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Novel metal-organic framework adsorbents for efficient storage of hydrogen

Periodic Reporting for period 1 - MOST-H2 (Novel metal-organic framework adsorbents for efficient storage of hydrogen)

Reporting period: 2022-06-01 to 2023-11-30

Widespread use of hydrogen as an energy carrier is a key priority for the EU, in order to achieve its climate and energy transition targets. Hydrogen can be produced from renewable sources and stored as gas (compressed, cH2 or cryo-compressed, CcH2), liquid (LH2 at ~20K), chemically bound to liquid organic carriers, or solids or physically adsorbed onto porous materials. Chemical routes are associated with increased binding energies and require elaborate heat management schemes, while poor cyclability, slow kinetics and high desorption temperatures are important limitations from the application viewpoint. Physical processes like compression and liquefaction, although more advantageous, are energy intensive due to the required multi-stage compression (up e.g. to 700 bar) or cooling (down to ~20K). Moreover, these processes require the use of expensive composite or heavily insulated bulky containers but they are also connected with critical safety considerations as they may involve very high pressures or continuous H2 boil-off. The use of nanoporous (pore dimensions<100 nm) materials for H2 storage is based on physical adsorption, involving weak physical gas-solid interactions. Gas adsorption is a spontaneous, exothermic, dynamic gas-solid equilibrium process favored at low temperatures and medium pressures. Although the search for effective H2 adsorbents has been ongoing for at least two decades, there are still significant challenges in developing and deploying nanoporous materials suitable for H2 storage.

Among the broad range of hydrogen adsorbents that have been investigated to date, metal-organic frameworks (MOFs) that are light-weight, highly ordered, porous crystals formed by combining inorganic building units (metal ions or clusters) and organic linkers, are considered to have notable advantages over other porous solids.
Despite intense research efforts worldwide, the development of MOF-based H2 storage systems is an underdeveloped area of research, as there is a clear technological gap on how to identify suitable low-cost MOFs with the optimum combination of volumetric and gravimetric capacity but also negligible environmental footprint, and incorporate them into actual storage tanks. MOST-H2 directly addresses this challenge by mainly aiming at:
- Developing new MOFs with usable H2 storage capacities of at least 10 wt% and 50 g/L below 100 bar.
- Developing a cryo-adsorption H2 storage system delivering up to 500 g of H2.

In this context, advanced synthetic strategies and sophisticated computational techniques, including machine learning, are being combined to deliver new, sustainable-by-design MOF adsorbents with suitable properties that can lead to more efficient, intrinsically safer and cost-effective storage solutions, compared to conventional hydrogen storage technologies. An important part of the project is also being devoted to developing a cryo-adsorption storage tank, which will be properly tested in a TRL 5 environment. The project developments are also being coupled with full life cycle analysis and techno-economic assessment of the MOST-H2 technology to assess its potential with a view to selected end uses (rail and road applications).
The main results achieved in the first 18 months of the project can be highlighted as follows:

- Construction and evaluation of the MOST-H2 database comprising approximately 10,000 MOF materials with specific topologies (targeted by the project), surpassing to a large degree the initial aim to create a database of 1,000 materials.
Machine Learning (ML) techniques were employed to predict the hydrogen adsorption properties of such MOF structures. These models were trained using large databases, and an iterative and self-consistent approach was adopted to improve the accuracy of the predictions and reduce the need for laborious and expensive experimental studies. The trained Machine Learning models will be further utilized to predict the top-performing materials in the MOST-H2 database in terms of hydrogen working capacity.

- Lab-scale development of a good number of novel, highly porous MOFs variances with hierarchical porosity and hydrogen sorption capacities (both gravimetric and volumetric) very close to the technical targets of the project, demonstrating a strong application potential.

- Investigation of alternative and greener MOF synthesis routes to facilitate upscaled material production in the form of monoliths.

- Investigation of different approaches for improved thermal management at system level through the development of MOFs/graphite flakes composites.

- Development of heat and mass transfer models to facilitate the design of the actual hysrogen storage tank.

- Initial life cycle assessment of the synthesis of MOF materials relevant for the scope and objectives of the MOST-H2 project using literature data but also experimental laboratory results.
The work so far mainly with respect to the material development activities has already led to important achievements going well beyond the State of the Art as:
(a) A novel family of MOF materials has been developed, offering new concepts in MOF crystal engineering. The versatile nature of these structures opens the door for combining diverse building blocks and thus offering structures never considered in the past. For this reason a pertinent patent application has been filed.
(b) The production of MOF monoliths in larger scale is expected not only to lead to reduced material cost but paves the way to industrial production and extended utilization on such material.